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Posts Tagged ‘SDSS’

The largest map of dark matter made with direct measurements, unveiled today by two teams of physicists at the U.S. Department of Energy’s Fermilab and Lawrence Berkeley National Laboratory (Berkeley Lab) removes a key hurdle for tracing the history of dark energy in the universe using ground-based telescopes.

This work done by members of the Sloan Digital Sky Survey collaboration points to greater successes for upcoming sky surveys, including the Dark Energy Survey, which will turn on the Dark Energy Camera on the Blanco Telescope later this year, and then the Large Synoptic Survey Telescope and the HyperSuprimeCam survey.

To find and map the invisible dark energy and dark matter that make up about 96 percent of the universe, physicists look at their effects on the matter and radiation we can see, namely galaxies.

Surveying galaxies from Earth-based telescopes is cheaper than satellite-based experiments but had traditionally had the drawback of having to make due with a less clear view of the sky. The same atmospheric distortions that make stars twinkle blurs attempts to track invisible dark matter in the universe made by measuring the distortion of background galaxy shapes, a process called weak lensing. DES and LSST will use this technique to create the largest galaxy surveys ever, covering more than one-eighth of the sky.

Layering photos of one area of sky taken at various time periods, a process called coaddition, can increase the sensitivity of the images six fold by removing errors and enhancing faint light signals. The image on the left show a single picture of galaxies from the SDSS Stripe 82 area of sky. The image on the right shows the same area with the layered effect, increasing the number of visible, distant galaxies. Credit: SDSS.

Layering photos of one area of sky taken at various time periods, a process called coaddition, can increase the sensitivity of the images six fold by removing errors and enhancing faint light signals. The image on the left show a single picture of galaxies from the SDSS Stripe 82 area of sky. The image on the right shows the same area with the layered effect, increasing the number of visible, distant galaxies. Credit: SDSS.Particle physicists and astronomers from Fermilab and Berkeley Lab have demonstrated a new technique for weak lensing that lessens the blurriness and allows researchers to see fainter galaxies, providing a younger picture of the universe. The two teams essentially layered snap shots of these distorted galaxies, in a process called coaddition, to remove errors caused by equipment or atmospheric effects and to enhance very faint light signals coming from deep in the universe.

Both teams depended upon extensive databases of cosmic images collected by the Sloan Digital Sky Survey, SDSS, which were compiled in large part with the help of Berkeley Lab and Fermilab.

“These results are very encouraging for future large sky surveys. The images produced lead to a picture of the galaxies in the universe that is about six times fainter, or further back in time, than is available from single images,” says Huan Lin, a Fermilab physicist and member of SDSS and DES.

Surveys of galaxies across large swaths of the sky track how clumps of dark matter have changed over time as dark energy exerts its repulsive push on them. Clumps of dark matter not only distort the images of galaxies behind them, but they determine how galaxies cluster around them. By combining this information with redshift data, the observed change in the color of light emitted by a star or other celestial object that is moving away from Earth, it’s possible to trace how the distribution of matter in the universe has evolved over time, offering insight into the growth of dark energy.

Researchers hope this new tool will help answer one of the largest questions for upcoming dark energy surveys and in cosmology: whether dark energy is what Einstein called a “cosmological constant”, a counterbalance to gravity’s pull on matter? Or is it something else such as gravity behaving differently at cosmic scales. The variation or lack of separation between clusters of galaxies and within the clusters across time will lead to new insight into this question.

To build one of the largest maps of dark matter and track its evolution across eras, the teams looked at two manifestations of gravitational lensing: those caused by large galaxy clusters and those caused by the overall distortion spread across the large scale structure of the universe. This second effect is called cosmic shear. Both of these distortions are caused by the gravitational fields of clumps of dark matter acting as lenses, bending the light from galaxies behind them. This distorts the shapes of these distant galaxies, making them look more elliptical. By measuring the ellipticities, or amount of distortion, physicists can infer properties of the dark matter, such as its abundance and how clumpy it is and the masses of the clusters.

“This image correction process should prove a valuable tool for the next generation of weak-lensing surveys,” Lin says.

— Tona Kunz

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Top left image shows SDSS-III's view of a small part of the sky, centered on the galaxy Messier 33. The middle top picture is a zoomed-in image on M33, showing the spiral arms of this galaxy, including the blue knots of intense star formation. The top right-hand image shows a further zoomed-in image of M33 highlighting one of the largest areas of intense star formation in that galaxy. Credit: SDSS

The world’s largest, digital, color image of the night sky became public this month. It provides a stunning image and research fodder for scientists and science enthusiasts, thanks to the Sloan Digital Sky Survey, which has a long connection to Fermilab.

Oh, yeah, and the image is  free.

The image, which would require 500,000 high-definition TVs to view in its full resolution, is comprised of data collected since the start of the survey in 1998.

“This image provides opportunities for many new scientific  discoveries in the years to come,” said Bob Nichol, SDSS-III scientific spokesperson and professor at University of Portsmouth.

Fermilab oversaw all image processing and distribution of data to researchers and the public from 1998 through 2008, for the first seven batches of data. These batches make up a large chunk of the ground-breaking more than a trillion-pixel image. The eighth batch of raw, reduced data, which was released along with the image at the 17th annual meeting of the American Astronomical Society in Seattle was processed by Lawrence Berkley National Laboratory. LBNL, New York University and Johns Hopkins University distributed that data. Fermilab’s SDSS collaboration members now focus solely on analysis.

“This is one of the biggest bounties in the history of science,” said Mike Blanton, professor from New York University and leader of the data archive work in SDSS-III, the third phase of SDSS.  “This data will be a legacy for the ages, as previous ambitious sky surveys like the Palomar Sky Survey of the 1950s are still being used today. We expect the SDSS data to have that sort of shelf life.”

The release expands the sky coverage of SDSS to include a  sizable view of the south galactic pole. Previously, SDSS only imaged small, spread out slivers of the southern sky. Increasing coverage of the southern sky will aid the Dark Energy Survey and the Large Synoptic Survey Telescope both southern sky surveys that Fermilab participates in.

Comparing the two portions of the sky also will help astrophysicists pinpoint any asymmetries in the type or number of large structures, such as galaxies. Cosmic-scale solutions to Albert Einstein’s equations of general
relativity assume that the universe is spherically symmetric, meaning that on a large enough scale, the universe would look the same in every direction.

Finding asymmetry would mean the current understanding of the universe is wrong and turn the study of cosmology on its head, much as the discovery of particles not included in the Standard Model would do for collider physics.

“We would have to rethink our understanding of cosmology,” said Brian Yanny, Fermilab’s lead scientists on SDSS-III. So far the universe seems symmetric.

Whether the SDSS data reveals asymmetry or not it undoubtedly will continue to provide valuable insight into our universe and fascinate amateur astronomers and researchers.

Every year since the start of the survey, at least one paper about the SDSS has made it in the list of the top 10 astronomy papers of the year. More than 200,000 people have classified galaxies from their home computers using SDSS data and projects including Galaxy Zoo and Galaxy Zoo 2.

In the three months leading up to the image’s release a record number of queries, akin to click counts on a Web page,  occurred on the seventh batch of data. During that time, 90 terabytes of pictures and sky catalogues were down loaded by  scientists and the public. That equates to about 150,000 one-hour long CDs.

Scientists will continue to use the old data and produce papers from it for years to come. Early data also works as a check on the new data to make sure camera or processing flaws didn’t produce data anomalies.

“We still see, for instance, data release six gets considerable hits and papers still come out on that in 100s per year,” Yanny said.

So far, SDSS data has been used to discover nearly half a billion astronomical objects, including asteroids, stars, galaxies and distant quasars. This new  eighth batch of data promises even more discoveries.

Fermilab passed the job of data processing and distribution on to others in 2008. The eight batch of data was processed by Lawrence Berkley National Laboratory and distributed by LBNL, New York University and Johns Hopkins University.

Fermilab’s four remaining SDSS collaboration members now focuses solely

illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS. Credit: Chris Blake and Sam Moorfield and SDSS.

on analysis. They are expected to produce a couple dozen papers during the next few years. The group touches on all of SDSS-III’s four sky surveys but focus mainly on the Baryon Oscillation Spectroscopic Survey, or BOSS, which will map the 3-D distribution of 1.5 million luminous red galaxies.

“BOSS is closest to our scientists’ interests because its science goals are to understand dark energy and dark matter and the evolution of the universe,” Yanny said.

For more information see the following:

* Larger images of the SDSS maps in the northern and southern galactic hemispheres are available here and here.

*Sloan’s YouTube channel provides a 3-D visualization of the universe.

*Technical journal papers describing DR8
and the SDSS-III project can be found on the arXiv e-Print server.

*EarthSky has a good explanation of what the colors in the images represent and how SDSS part of an on-going tradition of sky surveys.

*The Guardian newspaper has a nice article explaining all the detail that can be seen in the image.

— Tona Kunz

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